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Australopithecus sediba: a possible human ancestor

Australopithecus sediba is a possible human ancestor discovered in South Africa in 2010. The discovery was made at Malapa, a fossil-bearing cave located about 15 km (9.3 miles) NE of the well-known South African hominid-bearing sites of Sterkfontein and Swartkrans and about 45 km (28 miles) NNW of Johannesburg (Berger, et al., 2010). It is situated within the Cradle of Humankind World Heritage Site. The recovery effort was led by Lee Berger, a paleoanthropologist at the University of the Witwatersrand, Johannesburg. The find was made when Matthew, Lee’s 9 year old son, discovered hominin collar bone embedded in a rock (Balter, 2010).
The find comprised two extremely well-preserved partial skeletons that were initially thought be somewhere between 1.78 and 1.95 million years old (Dirks, et al., 2010), later revised to 1.977 million years (Pickering, et al., 2011). These belonged to a juvenile male (MH1) aged 12 to 13 at time of his death and an adult female (MH2) (Berger, et al., 2010). They were found together buried in alluvial sediment, deep within the Malapa cave, part of an eroded cave system. Also found were the remains of wildcats, hyenas and a number of other mammals. On the ground above the cave are a number of ‘death traps’, or long vertical shafts. The smell of damp issuing from the shaft would have attracted animals. The pair – possibly mother and son – may have fallen to their deaths while searching for water. The sediments imply that subsequent high-volume water inflow, perhaps the result of a large storm, caused a debris flow. This carried the still partially articulated bodies deeper into the cave, to deposit them along a subterranean stream (Dirks, et al., 2010).

MH1 and MH2 were assigned to a new australopithecine species, Australopithecus sediba. The word ‘sediba’ means ‘fountain’ or ‘wellspring’ in the Sotho language. The more complete cranium of the juvenile MH1 has a capacity of 420cc, probably at least 95 percent of adult size. The remains share numerous similarities with Australopithecus africanus in the cranial vault, facial skeleton, lower jawbone and teeth, but there are also significant differences in the cranial, dental and postcranial anatomy. Homo-like features include smaller molars and premolars and less pronounced cheekbones. Certain features of the pelvis are similar to those seen in Homo erectus. The lower-to-upper limb bone proportions are also similar to those of later Homo, and unlike the more apelike proportions of Homo habilis. The anatomy of its hip, knees and ankles suggest that Australopithecus sediba was a habitual biped. Overall, it was claimed that Australopithecus sediba shares more derived features with early Homo than it does with other australopithecines. However, Berger was reluctant to place the new discovery within Homo, preferring to classify it as an australopithecine (Berger, et al., 2010).

The initial announcement of Australopithecus sediba attracted extensive news coverage, but not everybody was convinced by the claims made for it. Australian anthropologist Darren Curnoe was reported (MacKnight, 2010) as claiming that Australopithecus sediba is in the wrong place at the wrong time to be a human ancestor. He noted that Homo habilis emerged in East Africa well before the time of Australopithecus sediba. However, his argument does assume that Homo habilis is indeed an early human. This may not be the case. It is also possible that at least some of Australopithecus sediba’s humanlike features could have evolved independently, and may not necessarily imply shared ancestry (Wood & Harrison, 2011).

Nevertheless, subsequent studies do support Berger’s initial claims. They suggest that aspects of the brain, dental morphology, pelvis, hand and foot of Australopithecus sediba could be interpreted as incipient humanlike features. A virtual endocast of the brain, obtained from synchrotron scanning, revealed an australopithecine-like size and pattern of convolutions. However, the orbitofrontal region showed possible development towards a humanlike frontal lobe. Possibly some neural reorganization of the brain preceded its later size increase in early humans (Carlson, et al., 2011).

The teeth of MH1 and MH2 are a mosaic of primitive and derived traits. Cladistic analysis of 22 dental traits suggest that Australopithecus sediba was a sister species of Australopithecus africanus (i.e. the two shared a common ancestor) and that the two were further evolved in the direction of Homo than were the australopithecines from East Africa (Irish, Guatelli-Steinberg, Legge, de Ruiter, & Berger, 2013). The lower jawbone morphology reduced dentition (especially canines and premolars) confirms that Australopithecus sediba was a distinct species to Australopithecus africanus and not merely a late-surviving form of that species (de Ruiter, et al., 2013).

The upper ribcage of Australopithecus sediba exhibits an apelike funnel shape, unlike the barrel shape associated with Homo. The funnel shape, as noted above, may be an adaptation to under-branch suspensory locomotion. The barrel shape may be associated with the increased chest volume and lung function necessary for endurance walking and running. The lower thorax, however, appears less flared than that of apes and more closely approximates the morphology found in humans (Schmid, et al., 2013). The spine is long and flexible, a form that has more in common with early Homo than with other australopithecines. Curvature of the lower spine is a hallmark of walking upright (Williams, Ostrofsky, Frater, Churchill, Schmid, & Berger, 2013).

The upper limbs were still predominantly apelike, suggesting the retention of substantial climbing and suspensory abilities (Churchill, et al., 2013). The hands show a mixture of australopithecine and human features. They retained adaptations for tree-climbing, but there was also a long thumb and shorter fingers. These suggest precision gripping of the type associated with tool manufacture and use (Kivell, Kibii, Churchill, Schmid, & Berger, 2011).

The pelvis and foot presented a mosaic of apelike and humanlike characteristics. These suggested adaptations to a more efficient (albeit not entirely human) form of bipedalism, at the expense of reduced arboreal efficiency (Kibii, et al., 2011; Zipfel, DeSilva, Kidd, Carlson, Churchill, & Berger, 2011). The bipedal mechanics differed from those reconstructed for other australopithecines, suggesting that there may have been several forms of hominin bipedalism at this time. The adaptations of Australopithecus sediba may have enabled it to both walk and climb reasonably well and thus survive in a dual arboreal/terrestrial world (DeSilva, et al., 2013).